Revert to simple gt-mayor/gt-deacon session names

Reverts the session naming changes from PR #70. Multi-town support
on a single machine is not a real use case - rigs provide project
isolation, and true isolation should use containers/VMs.

Changes:
- MayorSessionName() and DeaconSessionName() no longer take townName parameter
- ParseSessionName() handles simple gt-mayor and gt-deacon formats
- Removed Town field from AgentIdentity and AgentSession structs
- Updated all callers and tests

Generated with Claude Code

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/mail/router.go

@@ -13,7 +13,6 @@ import (
 	"github.com/steveyegge/gastown/internal/config"
 	"github.com/steveyegge/gastown/internal/session"
 	"github.com/steveyegge/gastown/internal/tmux"
-	"github.com/steveyegge/gastown/internal/workspace"
 )
 
 // ErrUnknownList indicates a mailing list name was not found in configuration.
@@ -936,13 +935,7 @@ func (r *Router) GetMailbox(address string) (*Mailbox, error) {
 // Uses send-keys to echo a visible banner to ensure notification is seen.
 // Supports mayor/, rig/polecat, and rig/refinery addresses.
 func (r *Router) notifyRecipient(msg *Message) error {
-	// Get town name for session name generation
-	var townName string
-	if r.townRoot != "" {
-		townName, _ = workspace.GetTownName(r.townRoot)
-	}
-
-	sessionID := addressToSessionID(msg.To, townName)
+	sessionID := addressToSessionID(msg.To)
 	if sessionID == "" {
 		return nil // Unable to determine session ID
 	}
@@ -959,21 +952,15 @@ func (r *Router) notifyRecipient(msg *Message) error {
 
 // addressToSessionID converts a mail address to a tmux session ID.
 // Returns empty string if address format is not recognized.
-func addressToSessionID(address, townName string) string {
+func addressToSessionID(address string) string {
 	// Mayor address: "mayor/" or "mayor"
 	if strings.HasPrefix(address, "mayor") {
-		if townName != "" {
-			return session.MayorSessionName(townName)
-		}
-		return "" // Cannot generate session name without town name
+		return session.MayorSessionName()
 	}
 
 	// Deacon address: "deacon/" or "deacon"
 	if strings.HasPrefix(address, "deacon") {
-		if townName != "" {
-			return session.DeaconSessionName(townName)
-		}
-		return "" // Cannot generate session name without town name
+		return session.DeaconSessionName()
 	}
 
 	// Rig-based address: "rig/target"